The present invention pertains to a portable engine, and, in particular, to a single cylinder internal combustion engine of the size and type adapted for use in power equipment such as that used in lawn and garden, general utility and snow removal operations. Such equipment includes but is not limited to lawnmowers, snow throwers, generators, string trimmers, leaf blowers, ice augers, earth movers, etc.
A variety of portable engines which are relatively lightweight have been employed with outdoor or lawn and garden power equipment such as lawnmowers, string trimmers and the like. While both four cycle and two cycle engine designs have previously been utilized, four cycle engines have generally emerged as the preferred design from the standpoint of reducing exhaust and noise emissions. In particular, recent legislation has reduced allowable exhaust emission levels to a point where the engine must he carefully designed to comply with promulgated emission levels, and four cycle engines typically burn cleaner than two cycle engines.
One shortcoming of some commercially available four cycle engines that undesirably leads to higher emissions relates to their propensity to distort in shape. As the engine heats up during usage, the thermal expansion of the engine cylinder block components may produce bore distortions which allow leakage, such as lubricating oil, to pass the piston rings and pollute the engine exhaust. In particular, due to weight and space restrictions inherent in the utilization of these portable engines, and in order to accommodate other mechanical workings of the engines such as drive components for an overhead camshaft, the cylinder bore wall thickness may vary markedly around the bore perimeter. In addition, the walls may be less rigid than optimal because a thin inner wall must be provided to separate multiple internal chambers. In addition, reinforcing ribbing may be withheld due to spacing requirements. These wall thickness variations and lack of rigidity may result in a non-uniform expansion or distorting of the cylinder bore during combustion pressure and thermal cycling, and consequently an unclean engine combustion may occur. A further consequence of such distortion producing leakage is to form oil-based deposits in the combustion chamber. It is well known that these deposits are an important source of the emission of volatile organic compounds, a critical constituent in the control of exhaust emissions. Build-up of these deposits over time is the main contributor to the deterioration of the control of exhaust emissions over the useful life of an engine.
Another potential source of cylinder bore distortion stems from the use of a separate head and cylinder. When a cylinder head is fastened to the cylinder block, the point loading around the cylinder bore which occurs with head bolt torquing may create sufficient bore distortion to compromise the seal with the piston. The head gasket normally introduced between the cylinder and head creates additional bore distortion concerns. For example, because the head gasket serves as a heat transfer barrier and thereby does not uniformly distribute the heat energy over the cooling surfaces of the engine, distortion potential of the cylinder bore associated with thermal expansion may be exacerbated.
The need for flywheels introduces other problems in portable engines. Due to space constraints, flywheels are typically mounted on the crankshaft at a position external of the engine housing and in a cantilevered fashion. To support this cantilevered flywheel mass without failure, the crankshaft must be formed with a stronger shaft than would be required without an external flywheel. Regardless of whether this stronger shaft is obtained by using a stronger material or by providing a larger diameter shaft, the overall weight of the engine is likely to be increased, and the ease of portability of the engine is thereby diminished. In addition, flywheels are frequently formed separately from the crankshaft and then rotatably fixed together via keying. Unfortunately, during aggressive or emergency stopping which can occur by accident or by use of braking devices, the inertia of the flywheel can lead to breakage of the key between the crankshaft and the flywheel, which renders the engine nonoperational.
Thus, it is desirable to provide a small internal combustion engine which overcomes these and other disadvantages of prior art engines.